"Remarks—Climatic impact
It is interesting that the mean optical depth found in this case (0.35) corresponds to that found in climatological satellite measurements by Ponater et al. (2002) and Minnis et al. (2004), and that the microphysical properties are consistent with a wide range of observations and models by prior authors. Accordingly, it is appropriate to speculate on their effect on climate.
The issue of the impact of contrail-generated cirrus (CS) on climate change has been treated by a number of investigators.
Sassen (1997) suggested that the unusually small particles typical of many persistent contrails might favor the albedo cooling over the greenhouse warming. Using a 2D mesoscale cloud model, Khvorostyanov and Sassen (1998) computed the distribution of the mean crystal radius, concentration, and ice water content of a contrail after 30 minutes of development.
They found a twofold effect. At the surface, the net greenhouse minus albedo effect was negative with a cooling of 15 Watts per square meter.
However, at the top of the atmosphere (corresponding to the entire atmospheric column), the net effect was a warming of 8 Watts per square meter.
We note that the latter simulation for the early stage of the cloud produced very large concentrations of small crystals, and that the longwave warming would be increased relative to the shortwave cooling, with the much larger particles, such as are found in the present study.
An evaluation of the effects of contrail cirrus over the globe is complex because of the great geographic and diurnal variability of air traffic, the variability of optical thickness and persistence of the clouds, the background brightness, and the natural changes in ambient humidity (Minnis 2003).
There are also a number of feedback mechanisms at play that climate models have not dealt with very well.
The finding that the vertical motions accompanying the transformation to cirrus also increase the humidity above the contrail is one such factor (Jensen et al. 1998). Nevertheless, using results from a general circulation model simulation of contrails, Minnis et al. (2004) found that the cirrus trends resulting from contrails in the United States are estimated to cause a tropospheric warming of 0.2°– 0.3°C per decade, a range that includes the observed trend of 0.27°C per decade between 1975 and 1994. One must emphasize that, even if correct, this is a regional effect.
We may summarize the various studies as follows:
1) regional effects in the 1990s in the United States and Europe have a cover of 0.5%–2% with a maximum over Europe of 0.35%, and warming of 0.1–0.2 W per square metre, and
2) global effects that are about 0.1 of the regional values.
These estimates are based upon the work of Minnis et al. (1998), Ponater et al. (2002), Palikonda et al. (2002), and Minnis et al. (2004).
The latter authors also project an aircraft scenario for 2050 that would produce a regional radiative forcing of 1.5 Watts per square metre over Europe, a global coverage of 0.5%, and a radiative forcing of 0.05 Watts per square metre. In short, the present-day effects are significant regionally but in the noise globally. Further research is necessary to assess factors such as the amount of cirrus that is initiated by the contrails but not distinguishable from natural cirrus.

Summary and Conclusions

This study of the transformation of contrails to cirrus uncinus (mares’ tails) was made possible by a fortuitous and unique combination of observations by a groundbased camera, a vertically pointing lidar, satellite imagery, and a new database of aircraft flight tracks. The photograph documented the metamorphosis of the contrails to cirrus and the formation of fallstreaks almost continuously along the contrails in directions consistent with the winds.
A novel method was developed to use the visible photograph with the lidar-measured cloud heights to obtain the true orientation of the contrails and fallstreaks, their spacing, and their dimensions.
Although the cirrus lines appeared to converge in the southwest, they were actually almost parallel to one another, oriented from about 232° to 52° and spaced about 4–5 km apart, as confirmed by the satellite image and by the time spacing of their passage over the lidar.
The flight tracks occurred in two corridors: the western corridor corresponded to flights from east coast cities to Atlanta and others in the general vicinity; the eastern corridor flights originated mostly from the same cities to destinations in Florida and the vicinity.
Although the flight tracks in each corridor followed nearly identical paths in sequence, their contrails separated in time and were advected to the southeast with the component of the wind normal to the trail. When the contrails passed over the lidar at GSFC each appeared as a cirrus uncinus cloud with a generator turret and fallstreak of ice crystals oriented nearly normal to the length of the contrail. Using the appropriate component of the wind to extrapolate backward in time, we were able to correlate each contrail at the lidar with a specific aircraft flight with a remarkable correlation coefficient of 0.99.
The older contrails from a few of the flights in the western corridor arrived at the lidar nearly simultaneously with the younger ones from the eastern corridor, thus producing broader contrail cirrus at the lidar. This is also manifested by the overlap of contrails in the eastern corridor as seen by the satellite. The lag between the initial formation and the time of first detection by MODIS is approx. 33 min. The 2 h required for the contrails from the western corridor to reach the lidar at GSFC is a measure of their minimum lifetime, because they persisted beyond GSFC. The lead author watched them for about 1–2 hours later in the afternoon.
Among other notable features of the contrails, which have formed at about -40°C, are the convective turrets or generator cells of the cirrus uncinus that grow to 1–2-km horizontal size from the initial downward pendants created by the wake dynamics of the aircraft, as shown by other investigators. The pendants are composed of a large concentration of tiny ice crystals and large ice water content (IWC) and grow via heating by longwave radiation from the ground.
The brightness of the generator cells, comparable to that of warm cumulus, is also a result of the large concentration of tiny ice particles, as deduced from the lidar observations and by in situ sampling by prior authors.
The lidar observations provided three time–height profiles:
1) the attenuated lidar scattering ratio (ALSR)
2) the extinction coefficient (d)
3) the radar reflectivity factor (Z)
The slope of the fallstreaks provided particle fall speeds and an approximation to the particle median volume diameters (D0). The values of d and D0 lead to the IWC and to the parameter K = (d/IWC), the ratio of extinction to IWC. All of the values deduced are in reasonable agreement with those by other investigators. The vertical integrals of d and IWC provide the optical thickness T and ice water path (IWP), respectively.
The time-averaged values (over the 93-min period observed by the lidar) are Tau = 0.352 and <IWP> = 8.14 grams per square meter, and are dominated by the fallstreaks.
The average ice water per meter along the length of the contrail is 16 Kilograms (35.4 pounds) per meter, some three to four orders of magnitude (1,000 to 10,000 times) greater than the water vapor released by typical jet aircraft, also similar to previously reported values.
The net effect of the water and particles released by the aircraft results in a major inadvertent modification of the atmosphere under appropriate ambient conditions. Furthermore, the evaporation of the fallstreaks at lower levels indicates the downward transfer of moisture from the upper levels where the crystals have grown.
Although most investigators favor the finding that contrail cirrus produce atmospheric warming on regional scales in the United States and Europe, the impact on global warming is still in the noise. Should aircraft activity increase as projected, the global effects would become significant by 2050."